IK Solver Won't Bend Knees Except Backwards -- Rolling Bones Not Helping, Constraints Not Helping
Thank you to everyone who responded.  A synopsis of the workaround.  (I say workaround because this isn't really a fix, but it's more than functional enough to unblock progress and move forward.  A true fix would be figuring out why the IK solver isn't deducing a solution to chains of length three.)

The original IK configuration had the constraint placed on the FOOT bone with an IK chain of length three.  The solution involved moving the IK constraint to the SHIN bone [one up] and decrementing the IK chain by one.  Now the knee bends properly.

I speculate (again SPECULATE, I don't know if there is any basis here) that perhaps having the foot as the target of the IK chain meant that there were two bends [the knee and the angle] which were being used for determining the direction of the bend -- that is to say, instead of pointing the knee bend at the knee pole, it was trying to point the ankle at the pole and ignoring the knee, keeping that segment strait.

I'm very grateful to everyone who provided insight.  Special kudos to Jerryno and Naioai for setting me on the right track.